Christina Edkins had only been the top deck of the number 9 First West Midlands service for a matter of minutes when she was attacked as it pulled into a stop in Birmingham city centre at 7.37am this morning.
The popular teenager had been on her way to school.
Emergency services rushed to the scene after the bus driver reported that a passenger had been wounded.
But despite frantic efforts by passengers and paramedics to resuscitate her, she was pronounced dead at the scene.
A 22-year-old man was arrested a short distance away on suspicion of her murder at 12.20pm, around five hours later, after a huge manhunt.
He was held in Harborne Road during a foot chase after officers spotted a man who fitted the description of the suspect acting 'furtively' at the back of Morrisons.
It has since emerged that Christina had tweeted her concerns about a man she felt was acting strangely on a bus just weeks before she was killed.
On February 8, she said: 'This man is worrying me on the bus keeps getting up and walking up and down stairs, and sitting in different place.'
Police said they will be examining Christina's 'social media footprint' as part of the investigation, but stressed at this stage they were treating the murder as a 'random attack'.
Detectives said the suspect was already on the bus when she got on board and that there was 'nothing immediately' to suggest the pair knew each other.
Officers are examining CCTV tapes on the bus which could have filmed the murder, but are not looking for anyone else in connection with her death.
Christina's headteacher at Leasowes High School in Halesowen, Neil Shaw, today led tributes to the teenager.
In an emotional statement outside the school, he said: 'Christina was a bright and popular student much loved by staff and students alike.
'The school is working closely with the police and a team of counsellors to provide support to our pupils and staff.'
Popular Christina revealed her ambitions to be a nurse and tweeted about how she 'hated' buses just weeks before her death.
Less than 24 hours before she died, Christina tweeted at 1.28pm on March 6: 'Don't need anyone to talk to about how I feel just need someone to be lazy with me and watch films and make me laugh.'
She also posted pictures of herself and her school friends in their uniforms during a French lesson.
FRIENDS TWEET TRIBUTES TO 'HAPPY AND SMILING' CHRISTINA
Jae-OiMySize wrote: 'R.I.P Christina your gone but ill never forget you'
marniedaisy, who claimed to have been best friends with Christina since the age of three, wrote: 'i just don't want to believe it, I'm literally heartbroken x'
sp3nnn- added: 'It won't be the same anymore Christina was always so happy and smiling. She had such a beautiful smile you couldn't help but smile back rip'
OllyRiley wrote: 'RIP @ChristinaEdkins we will never ever forget you, remember our promise up there okay! I won't break it'
KaraJns said: 'We had the best heart to heart on Saturday, won't forget it. So glad i met you. Rip beautiful @ChristinaEdkins'
farheenok tweeted: '@christinaedkins RIP, you were one of the nicest people ever, the good people always go first, we'll never forget you x'
On February 23, she tweeted: 'When I'm on bus and @KatieEllizabeth sends me something funny and I burst out laughing and people just stare at me, hahaha opsyy.'
On the same day, she tweeted: 'Eughh hate catching buses, can't wait to start driving.'
Two days later, she said: 'Want to be a nurse just cba to study for it all tbh'.
A tribute page has also been set up on Facebook for 'beautiful angel' Christina, with users calling her death 'a waste of a beautiful young life' and saying 'justice will prevail'.
Hannah Louise said: 'Such a pretty girl, tragic waste of life, thoughts are with her family so, so upsetting x.'
Vernon Williams described her death as 'very very sad news indeed', while Angel Sheppard said: 'Rest in peace thinking of all the family.'
Hema Trivedi posted: 'Rip beautiful girl! Heaven has gained yet another beautiful angel!'
Jade Sarah said: 'Such a sad day thoughts are with your family and friends your such a pretty girl rest in peace I lost a friend not so long ago who was stabbed what's the world coming to it was nothing like this before now the world's messed up with knife crime!!!'
Jules Jewels said Christina's death had left the 'nation in shock', writing: 'I don't know you Christina but i am so very sorry for what happened today.
'The whole nation is in total shock over this and my thoughts are with all of your friends and family at this most difficult time. God bless.'
Adam Willett said: 'Justice will prevail rest in peace...wrongly taken x my thoughts r with your family.'
Denise Henty described Christina's death as 'such a waste of a beautiful young life'.
Detective Superintendent Richard Baker, of West Midlands Police, said: 'This is a tragic case and the victim's family are devastated.
'We are still anxious to hear from anyone with information or who may have witnessed the incident this morning to speak to us via the dedicated hotline 0800 096 0095.
'There will be a high police presence across the city this afternoon following this morning's tragedy, particularly on buses and in schools in the area to provide reassurance to local people.'
Police officers were also on guard at the family's semi-detached house near Birmingham city centre.
In an appeal shortly after the attack, he had described the suspect as 'extremely dangerous' and feared he may still be carrying weapons.
He said they were looking for a black man in his late teens or early 20s who was wearing a black hoodie with a tiger or leopard motif on the back.
He said it was not known whether the suspect knew the victim.
Around three hours after the attack, a man was detained and spoken in Lower Temple Street in the centre of the city, but was released without being arrested.
Traffic backed up into the city centre after police closed off the busy Hagley Road while forensics experts spent several hours combing the bus and surrounding area for clues.
Shocked passersby took to Facebook to speak of their horror at the incident.
Darren Bent, from Birmingham, said his wife was on the bus.
He wrote: 'Can't believe some guy has just stabbed and killed a school girl right in front of my missus on the number nine bus on her way to work and she had to give CPR love you xxx the guy did it as bus was about to stop and ran off.'
Stephanie Clinton added: 'It doesn't make you a man if you carry a knife it makes you scum. It doesn't matter if you in a gang or not. It's all wrong so grow up and put the knife down.'
Twitter user Mareesha wrote: 'So the girl got stabbed by her friend?'
AustymZogs tweeted: '15yr old Schoolgirl stabbed to death by a friend on a bus in
Birmingham, UK during rush hour this morning.'
A bus user yesterday tweeted that a passenger was discovered carrying a knife on the service only yesterday.
The West Midlands Police spokesperson said: 'At 7.37am today, police received a call from a member of the public reporting that a passenger had been attacked.
'Officers and other emergency services were immediately dispatched but it was clear that nothing could be done to save the victim.
'West Midlands Police have identified the victim's family and are in the process of breaking the sad news.'